Investigating nanostructuring within imidazolium ionic liquids: a thermodynamic study using photochromic molecular probes.

Molecular photoswitches have been used to investigate the possibility of nanostructured polar and nonpolar domains in ionic liquids (ILs). Two photochromic compounds, spiropyran (BSP) and spirooxazine (SO) were added to imidazolium based ionic liquids containing the anion [NTf2](-), and their photochromic behavior was monitored with increasing side chain length (C(2)-C(12)) of the imidazolium cation. Increasing side chain length was found to have only minor effects on the rate of thermal relaxation of the merocyanine form of spiropyran (MC(BSP)) and spirooxazine (MC(SO)) to BSP and SO, respectively. BSP was found to be a suitable optical probe, as linear correlations in parameters were observed for this compound. This is believed to be because BSP-IL interactions are based on hydrogen bonding between the MC(BSP) and the ionic liquid cations, compared to MC(SO), which is limited to electrostatic interactions. Hence, the sensitivity of MC(BSP) is enhanced in the charged polar regions of the IL. Increasing the side chain of the cation results in slight increases in MC(BSP) to BSP relaxation activation energy from 96.93 kJ x mol(-1) in [C(4)mIm][NTf(2)] to 105.27 kJ x mol(-1) in [C(12)mIm][NTf(2)]. MC(BSP) to BSP relaxation DeltaS(double dagger) and DeltaH(double dagger) values also increase with increasing side chain length. The ability for spirocyclic compounds to switch between polar and nonpolar forms appears to allow polar and nonpolar regions in ILs to be probed dynamically using a single probe dye. It appears that the value of the ground state equilibrium constant, K(e), is dominated by the nonpolar regions of the IL while the equilibrium constant of activation, K(double dagger), is dominated by the polar regions. A correlation of side chain length to equilibrium constant of activation is believed to be because polar regions are possibly expanding due to increasing influence of nonpolar side chain interactions and compound insertion upon the overall solvent structure. The result of such reordering and dispersion of polar regions reduces solvent-solute interactions which increases the rate of MC(BSP) to BSP relaxation.